A straight swap, even with a little alteration to the flue, takes about a day. Then there's the next morning for commissioning and re-filling etc.
It all depends on the details. One thing to bear in mind is that, with a gas boiler, the size of the gas supply (the copper tube inside the house) may have to be increased. Regulations have changed during the past 17 years. Without a lot more detail, I can only suggest this as a possible extra.